WildAgain Wildlife Rehabilitation, Inc.

Milk replacers for wildlife
Kcal calculations - two different methods


The Atwater system (after Wilbur Olin Atwater) or derivatives of this system are used for the calculation of the available energy of foods. The system was developed largely from the experimental studies of Atwater and his colleagues in the later part of the 19th century and the early years of the 20th. Atwater collected values from the literature and also measured the heat of combustion of proteins, fats and carbohydrates. These vary slightly depending on sources and Atwater derived weighted values for the gross heat of combustion of the protein, fat and carbohydrate in the typical mixed diet. While all foods will have slightly different values, the accepted factors to use when calculating kcals are 4 kcals/gram of protein; 9 kcals/gram of fat; and 4 kcals/gram of carbohydrate.

For pet foods, the Association of American Feed Control Officials (AAFCO) prescribes the use of a modified Atwater system when calculating calorie values for food products for the petfood industry, based on a presumption that pet food is about 90% as digestible than human foods. As such, the modified Atwater factors are 3.5 kcals/gram of protein; 8.5 kcals/gram of fat; and 3.5 kcals/gram of carbohydrate. PetAg and other manufacturers use this approach.

During WildAgain’s testing and research, several nutritionists have suggested that the standard Atwater factors are more appropriate to estimate calorie values in powdered milk replacer formulas, as the ingredients, once fully reconstituted, should be very digestible. Both calculations have been included in all of WildAgain’s analysis, so that the user of the information can make their own determination of which calculated value to use.

Here's an example, using Esbilac® Lot #3419 and looking at the lab analysis from Midwest Labs. Carbohydrates are calculated by subtracting the percent values of protein, fat and ash from total solids to arrive at a remainder, which is carbohydrates.



Atwater calculation


Modified Atwater calculation

Solids 95.6%     Factor   Kcals/gram     Factor   Kcals/gram
Protein 38.8% .388 X 4 = 1.552 .388 X 3.5 = 1.358
Fat 42.2% .422 X 9 = 3.798 .422 X 8.5 = 3.587
Ash 5.6%                    
Carbs 9.0% .090 X 4 = .360 .090 X 3.5 = .315
Total           5.71         5.26

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